Conventionally, in various systems, a digital signal processing device such as a digital signal processor (DSP) is used. For example, in a vehicle-mounted audio system, a DSP that processes an audio signal is disposed. The DSP has a digital filter such as an infinite impulse response (IIR) filter. Various coefficients in the digital filter (generically referred to as “filter coefficients” hereafter) are adaptively set up in accordance with the state or the like of the system including the DSP.
There is a case in which when a DSP performs signal processing using a digital filter, the digital filter diverges or an abnormal state occurs in the DSP, and, as a result, there occurs a signal having a large amplitude exceeding a maximum allowable in a system including the DSP. Particularly, when a signal having a large amplitude occurs in a vehicle-mounted audio system, there is a possibility that a high-volume sound is outputted from a speaker and interferes with driving. Therefore, detection of the occurrence of a large amplitude and prompt suppression of a high-volume sound are required. As a typical method of detecting the occurrence of a large amplitude, there is a method including comparing the amplitude value of a signal with a preset threshold having a fixed value (for example, refer to Patent Literature 1).